Deprecation Warnings while (should => is_expected)

I had an following deprecation warning while using expect syntax:-

```ruby
context 'enmerize' do
    it { is_expected.to enumerize(:role).in(:student, :teacher).with_default(:student) }
 end
```

```ruby
`failure_message_for_should` is deprecated. Use `failure_message` instead. Called from /Users/Puru/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/enumerize-matchers-0.0.1/lib/enumerize/matchers/enumerize_matcher.rb:33:in `block in <top (required)>'.


If you need more of the backtrace for any of these deprecations to
identify where to make the necessary changes, you can configure
`config.raise_errors_for_deprecations!`, and it will turn the
deprecation warnings into errors, giving you the full backtrace.

1 deprecation warning total

```